ULTRATHERM 908i
30
gbo Medizintechnik AG 2004
Version 1.2
5.1.5. Oscillator
The oscillator operates at a voltage of 6.2 V measured across C219. The basis voltage of V208 is
about 3.6 V. The RF wave should not be measured at V208 because the oscillations can be
interrupted in case of loading; that’s why, it is measured at V209. By means of L210 the voltage is
tuned to the maximum of about 10 V
pp
to 13 V
pp
measured at the collector of V210. The core of
L210 shall not be displaced without checking the oscillations by a RF oscilloscope because the
oscillator operates only in a small tuning range of the core.
5.1.6. Driver
A voltage of 3.8 V measured across C233 ensures a sufficient zero-signal current of V209.
5.1.7. RF circuits at the final stage output
Spesial RF measuring instruments are required for matching the output circuits.
When the coils L226 or L227 are bend the resistors R250 and R251 can burn off. An auxiliary
matching is possible by temporarily clamping two LEDs each, which are connected in anti-parallel
to each other, in parallel to R250 and to R251. With small output powers these LEDs can be taken
to the extinguished mode by slightly bending the coils L226 and L227. The coil 223 can only be
adjusted by means of a spectrum analyser or wobbler. By slight bending it is adjusted to serial
resonance with C265/C266 at 54.24 Mhz. In case of after-sales service work the coils L219 to
L222 can only be checked for their geometry.
L219
L220
L221=222
Inner diameter [mm]
10
10
10
Mounting height on the PCB [mm]
22
22
22
Distance between turns [mm]
3,5
Length of the coil [mm]
8,5
8,5
5.1.8. Typical measurement values with a matched load (50 ohm)
U
in
(X207)
= +2.0 V
The output power is 100 W
U at X216
= +1.0 V
U at X217
= -3.5 V
U at X218
= -0.2 V
U at N201/3, pin 8
= +3.3 V
U at N201/4, pin 14
= +3.7 V
U at N201/1, pin 1
= +3.5 V
U at N201/2, pin 7
= +14.3 V
U at the basis of V210
= 3.1 V
